Thirty years ago this month, a young teacher called Blair Peach was killed during a demonstration against the National Front in Southall, west London. Peach was a member of the Socialist Workers Party and the Anti-Nazi League, which had organised the protest during an upsurge of support for the far right. Peach's death and his funeral, attended by the thousands who accompanied the procession through East London, was memorialised in art, poetry and song.

It was believed that he had been killed by a blow on the head from a police radio but the exact cause was never officially established. A jury returned a verdict of death by misadventure. The policing of the protest, in which the Metropolitan police's Special Patrol Group (SPG) were notably involved, remains a topic of debate to this day.

Another 30 years, another demonstration, another death, albeit one that appears to have no link at all to violence by either police or demonstrators. Once again, however, the policing of the protest is under scrutiny.

Of course police tactics have changed over the years. The outright confrontation, like an old Roman battle, which was common in the 70s and 80s in demonstrations, has been refined. The nightmare scenario, as far as the police are concerned, is a repeat of the poll tax riots of 1990 when control of the centre of London was lost. Everything is now done to try and avoid a repeat. The surveillance techniques offered by closed circuit television, sophisticated long-distance filming, computerised identification and improved riot gear, mean that the old street battles are less common and have been replaced by containment. What the City saw on Wednesday and, to a lesser extent, yesterday, is a distillation of all those new techniques.

When the main body of protesters arrived on Wednesday from four different directions at their planned destination of the Bank of England, they soon found themselves hemmed in from all sides by ranks of police. Requests to leave the area were refused. This is, in police terms, the "kettle". It is best known for having been used in the May Day protests at Oxford Circus in 2001, after which it became the subject of a civil action, brought by one of those contained and only finally resolved by the law lords (in the police's favour) in January this year.

The kettle has also been used often in other, smaller, less publicised protests. Many away football fans, forced to stay behind police lines for long periods of time after a game, will be familiar with it. But what is significant about its use this week is that it is now apparently being applied in a rigid, inflexible way - policing as video-game. Its use was predicted and justified by the former assistant commissioner (special operations) at the Met, Andy Hayman, in an article in the Times earlier this week. "Tactics to herd the crowd into a pen ... have been criticised before, yet the police will not want groups splintering away from the crowd," he wrote.

There were certainly people anxious to smash windows and cause some mayhem in the City on Wednesday. But they were far outnumbered by a playful, peaceful, harmless group of protesters, including rappers, sax-players, jugglers, spliff-rollers, students, CND campaigners, passers-by, and men dressed as police officers and wearing blue lipstick. For many of them the intention had been to come and make a lunchtime April Fool's Day protest against the City and the banking world's self-indulgence, as well as to air concerns about everything from climate change to homelessness. But when many wanted to leave the area, hardly any were allowed to.

"Don't ask us, ask the gaffer," was the response from police officers to people who wanted to leave and were puzzled that they could not. Gaffers seemed in short supply and none had, apparently, been allowed to use their own initiative in allowing who to release from the pen of police in which the protesters were corralled. There's an old police joke in which a constable injured in a riot staggers back through the ranks for treatment. Another officer comes to his aid. "Thanks, sarge," says the constable. "That's OK - but by the way, I'm a superintendent." "Blimey," replies the officer, "I didn't realise I was that far back."

Where were the supers this time, why were the crowd given no instructions as to where they should go or when? The area became a public lavatory as people unable to leave used the entrances to Bank underground station as a urinal. The containment was backed up at the Bank of England, first with mounted police and then with police dogs, ramping up tensions and fuelling further bloody confrontations.

As people were eventually allowed to leave the area, they were funnelled out down a pavement on Princes Street with a police officer grabbing them by the arm as though they were under arrest. One officer, asked why people were not allowed to leave under their own steam, replied: "They might fall over." People were then asked for their name and address and required to have a photo taken. Those who refused were put back in the pen.

As for the more obvious signs of destruction - the Royal Bank of Scotland had its windows smashed. Why no one had thought to board up a building with the RBS sign on it, as many other outfits had been boarded up, is unclear. As for the violent clashes that led to cracked heads and limbs - how much was inevitable and how much avoidable?

Certainly, the police had to put up with much abuse and missiles, although these were mainly plastic bottles and sprayed beer and cider. Certainly, some demonstrators were bent on aggro but, then again, so were some of the officers on Queen Victoria Street. But how much of the trouble as the day wore on could have been avoided by policing that didn't involve containment? And what does this mean for the future of protesting? Does this mean that anyone wanting to go on a demonstration in the future needs to be prepared to be detained for eight hours, photographed and identified? And how long, if such techniques continue, or are further refined, before the confrontations become bloodier?

The thing about kettles is that they do have a tendency to come to the boil.